There has been proposed an A/D conversion circuit of a type called time A/D (TAD) that includes a pulse circulation circuit (e.g., see Patent Literature 1). The pulse circulation circuit is configured by connecting delay units in a ring shape, the delay units delaying an input pulse signal by a delay time in accordance with a power source voltage and outputting the delayed signal. This A/D conversion circuit applies, as the power source voltage of the delay units, an analog input voltage that is to be A/D-converted, and counts the number of circulation of the pulse signal in the pulse circulation circuit, to obtain A/D conversion data on the basis of the counted value. The A/D conversion circuit of the TAD type has many advantages such as being able to be configured of a digital circuit element like a gate and having a relatively simple circuit configuration to achieve low cost.
However, in the A/D conversion circuit of the TAD type, A/D conversion data does not have favorable linearity (having nonlinearity). This is because a transmission characteristic of the pulse circulation circuit (the relation between the power source voltage and the number of circulation per unit time) is a characteristic that can be approximated not by a linear function but by a quadratic function. Accordingly, in the configuration of Patent Literature 1, pulse circulation circuits of two systems are combined to completely offset the above nonlinearity and ensure the linearity of the A/D conversion data.
When an attempt is made to constitute a sensor product for outputting a digital signal by combining a sensor element having the nonlinearity and an A/D conversion circuit where the linearity of A/D conversion data is favorably maintained as described above, the following problem occurs. That is, in this case, the nonlinearity of the sensor element appears as it is in A/D conversion data outputted from the A/D conversion circuit. This produces the need for correcting the nonlinearity included in the A/D conversion data to the linearity by digital operation or the like. As a result, a response time increases by the time required for the above correction operation and it is thus difficult to achieve fast responsiveness as the sensor device.